Which organelle is responsible for storage of food particles, waste and water?

1.2 Vacuoles Have Multiple Functions

The vacuole is enclosed by a membrane, called the tonoplast. The number and size of the vacuoles in different plant cells vary greatly. Young cells contain a larger number of smaller vacuoles but, taken as a whole, occupy only a minor part of the cell volume. When cells mature, the individual vacuoles amalgamate to form a central vacuole (Figs. 1.1 and 1.2). The increased volume of the mature cell is due primarily to the enlargement of the vacuole. In cells of storage or epidermal tissues, the vacuole often takes up almost the entire cellular space.

An important function of the vacuole is to maintain cell turgor. For this purpose, salts, mainly from inorganic (e.g., PO43−, NaCL) and organic acids (e.g., amino acids, malate), are accumulated in the vacuole. The accumulation of these osmotically active substances (compatible solutes) draws water into the vacuole, which in turn causes the tonoplast to press the protoplasm of the cell against the surrounding cell wall. Plant turgor is responsible for the rigidity of non-woody plant parts. The plant wilts when the turgor decreases due to lack of water. Vacuoles also serve as a reservoir for protons and metabolically important ions (e.g., Ca2+) and maintain pH and ionic homeostasis.

They have an important function in recycling those cellular constituents that are defective or no longer required. Vacuoles contain hydrolytic enzymes for degrading various macromolecules such as proteins, nucleic acids, and many polysaccharides. Structures, such as mitochondria, can be transferred by endocytosis to the vacuole and are digested there, and are also referred to as lytic vacuoles. The resulting degradation primary metabolism products, such as amino acids and carbohydrates, are provided to the cytosol of the cell. This is especially important during senescence (Chapter 19.2) when prior to abscission, parts of the constituents of the leaves are mobilized to support the propagation and growth of seeds.

Vacuoles also function as waste deposits. With the exception of gaseous substances, leaves are unable to rid themselves of waste products or xenobiotics such as herbicides. Therefore, these compounds are ultimately deposited in the vacuole (Fig. 13.6).

Vacuoles of many plant cells (e.g., flower petals and fruits) contain anthocyanins (Chapter 18.3) and other pigments which attract pollinators and seed disperser, respectively. Accumulation of a variety of toxic compounds (e.g., phenolic compounds, alkaloids, cyanogenic glycosides; Chapters 16–18Chapter 16Chapter 17Chapter 18) and protease inhibitors in vacuoles discourage insects and animal herbivores, cell-wall-degrading enzymes (e.g., chitinases, glucanases, saponins) destroy pathogenic fungi and bacteria, and latex with wound-clogging properties serves as antiherbivory agents.

In addition, vacuoles also have a storage function. Many plants use the vacuole to store reserves of nitrate and phosphate. Some plants store malic acid temporarily in the vacuoles in a diurnal cycle (Chapter 8.4). Vacuoles of storage tissues contain carbohydrates such as sucrose and glucose (Chapter 9) and various forms of storage proteins (Chapter 12). Many plant cells contain different types of vacuoles (e.g., lytic vacuoles and protein storage vacuoles next to each other). The storage function of vacuoles plays a role when utilizing plants as natural protein factories. Genetic engineering allows the expression of economically important proteins (e.g., antibodies) in plants, where the vacuole storage system serves as a cellular compartment for accumulating high amounts of these proteins.

2.3.1 Mechanisms of sulfate transport across tonoplast

Vacuoles serve as storage compartments of sulfate. At the tonoplast membranes, proton-ATPase and proton-pyrophosphatase continuously pump up protons from cytoplasm to vacuolar lumen, providing inside positive electrical potentials (Martinoia et al., 2000, 2007) (Fig. 4.1). Under such circumstances, sulfate can be transported into vacuoles considerably through a tonoplast-localized ion channel or carrier as the electrical gradient is favorable for the incorporation of negatively charged ions. The actual mechanisms or transport proteins mediating the influx of sulfate to vacuoles is still unverified; however, studies with isolated mesophyll vacuoles indicate biphasic kinetics for the influx of sulfate to the vacuoles, suggesting existence of saturable and linear components (Kaiser et al., 1989). With respect to the efflux of sulfate from the vacuoles, the situation can be similar with the uptake of sulfate across the plasmamembrane, as sulfate being transported against the membrane potential, although the concentration of sulfate is normally high in the lumen side. A steep proton gradient generated by proton-ATPase and proton-pyrophosphatase can be used as a motive force for the efflux of sulfate from the vacuoles (Fig. 4.1). Alternatively, anion exchange systems may facilitate the influx and efflux of sulfate across the tonoplast.

(c) Vacuoles

Plant vacuoles are membrane-bound bodies of variable size and number within the protoplast. Within many cells, up to 90% of the cell volume may be occupied by a single vacuole or multiple vacuoles. In meristematic tissue, several smaller vacuoles (provacuoles) fuse together as the cells mature to produce a large vacuole. The surrounding single membrane, the tonoplast, pushes the cytoplasm against the plasma membrane as a result of high turgor within the vacuole. The solute potential in the vacuole promotes water uptake, causing cell rigidity and promoting cell enlargement. The smaller vacuoles contain some oxidoreductases which are typical of the membranes of the endoplasmic reticulum (ER). This led to the suggestion (Matile, 1975, 1978) that these particles originate from the ER and further development and differentiation give rise to fully developed vacuoles.

Solutes in vacuoles may be present in high concentrations (0.4–0.6 m) and include inorganic ions such as sodium, potassium, calcium, magnesium, chloride, sulphate and phosphate. In some CAM (crassulacean acid metabolism) plants, for example in Bryophyllum, all malic acid is present in the vacuole. Citrus fruits store approximately 0.3 m citric acid in vacuoles which can be squeezed out as juice. This compartmentation, therefore, prevents many important enzymes from being denatured. The movement of substances from the cytoplasm into vacuoles takes place by active permease-mediated transport and by fusion of membrane-bound vesicles with the tonoplast. Also stored in the vacuoles are: TCA (tricarboxylic acid) cycle acids; phenolic compounds such as flavonoids and tannins; amides; amino acids, peptides and proteins; betalains and alkaloids; gums and carbohydrates such as sucrose. These substances may occur as aqueous solution, amorphous deposits or crystals (e.g. calcium oxalate).

In addition to the turgor function of the vacuole it also serves as a storage organelle. Some storage substances are waste products of the cell which have been excreted into the vacuole from the surrounding cytoplasm. Other stored substances serve as food (energy) reserves, for example, proteins in the form of globulins, carbohydrates and phosphate in the form of phytin which comprises insoluble salts of phytic acid (inositol hexaphosphoric acid). Flavonoid deposits in vacuoles, for example in flower petals, assist in pollination by insects and poisonous alkaloids, on the other hand, repel predators. In some plants, the endospermic vacuoles (oleosomes) contain large reserve lipid deposits which are mobilized during seed germination.

Vacuoles are also rich in hydrolytic enzymes and therefore function in a similar manner to animal lysosomes. The enzymic activities are capable of hydrolyzing proteins, carbohydrates, lipids reserve phosphates and nucleic acids. They also mobilize the cytoplasmic organic matter incorporated by phagocytosis. Disruption of the vacuoles liberates the enclosed enzymes into the cytoplasm and consequently causes degeneration of intracellular components. The aleurone grains or protein bodies of seeds also display lysosomal properties. The levels of the hydrolytic activities rise during seed germination.

GNE-mutated disease – Nonaka distal myopathy

Another type of autosomal recessive distal dystrophy with onset in early adulthood but with weakness typically in the anterior compartment, causing foot drop, was reported as distal myopathy with rimmed vacuoles (DMRV) in Japanese patients by Nonaka and coworkers (1981). Progression to posterior compartment and proximal muscles with major disability occurred within 10–15 years after onset. Intrinsic muscles were also involved. DMRV is now considered the same disease as autosomal recessive quadriceps-sparing myopathy – hereditary inclusion body myopathy (HIBM).